Cyanopyridines are important starting materials for the production of pharmaceutical intermediates and other compounds. 3-methylpyridine (3-picoline) is an intermediate in the industrial production of nicotinic amide and nicotinic acid, which is an essential vitamin of the vitamin B-complex (vitamin B3).
Methods for the production of cyanopyridines from methylpyridines are known in the art. Commonly, the cyanopyridines are oxidized in the presence of a catalyst with ammonia and oxygen. The process is referred to as “ammoxidation” or “oxidative ammonolysis”. Various catalysts are known, which comprise specific combinations of catalytic components, which can be coated on carrier materials.
WO 03/022819 discloses methods for the production of heteroaromatic nitriles by ammoxidation of the corresponding alkyl-substituted pyridines. Methods and catalysts for oxidative ammonolysis of alkylpyridines are also disclosed in WO 95/32055.
After the ammoxidation reaction, a gaseous mixture is obtained, which comprises cyanopyridine, ammonia, residual methylpyridines, side products such as pyridine and gases of the reaction stream, such as oxygen, nitrogen, carbon dioxide and water. It is thus necessary to isolate the cyanopyridine from this mixture. Various methods are known in the art to separate the product from the other components.
In the art, methods for isolating the cyanopyridine are known, in which the cyanopyridine is quenched or extracted with an organic solvent.
U.S. Pat. No. 2,861,299 discloses a method for obtaining cyanopyridine from a reaction product, in which the reaction product is passed through a cooling condenser, a dry ice-catcher and a glass wool filter and extracted in a collector using an inert solvent such as benzene. The extraction with benzene as a quenching agent is also disclosed in U.S. Pat. No. 3,929,811. However, the use of organic quenching agents is disadvantageous, because organic solvents such as benzene are relatively expensive, toxic and inflammable at higher temperatures. Further, quenching methods are often complicated and require a large number of process steps.
In order to overcome the problems associated with quenching with organic solvents, methods have been developed in the art in which the use of an organic solvent is not necessary. CN101045706 A discloses a method in which the gaseous product obtained from the ammoxidation reaction is brought into contact with a circulatory aqueous solution in two absorption towers, in order to obtain an aqueous solution of 3-cyanopyridine. Since cyanopyridine is hydrolyzed to nicotinic acid at elevated temperatures and at a high concentration, it is necessary to control the concentration of 3-cyanopyridine in the absorption towers and in the product below 10 wt. %. Further, it is necessary to control the temperature of the circulatory aqueous solution, and thus the temperature in the two absorbent towers, to below 50° C., preferably between 15 and 30° C. When choosing such a low concentration and temperature, more than 95% of the product is recovered. However, the concentration of the product in the final solution is relatively low and it would be desirable to obtain the product at a higher concentration. Further, the loss of 5% cyanopyridine by hydrolysis is still relatively high. The gaseous product, from which cyanopyridine was separated, is not reused in the process and the process requires at least two absorption towers. For reasons of efficiency and also environmental reasons, it would be desirable to make further use of the gaseous product.
In general, the handling of the gaseous phase obtained in the production of cyanopyridines is problematic, because solid deposits tend to accumulate in the devices. Thus especially the reuse of such gaseous phases after depletion of cyanopyridines is problematic. Upon forming of deposits in pipes and other parts of the process plant, the transfer of gases can be reduced. Uniformity of the process cannot be maintained and the efficiency is decreased. The devices have to be cleaned in a time and labour consuming manner, during which the process has to be interrupted.